PROGRESS IN PLANETARY STRUCTURAL GEOLOGY ILLUSTRATED BY EXAMPLES FROM MARS

Faulting and brittle deformation of the Martian crust and surface is well documented by extensive arrays of structures. Despite several decades of study, many of these textbook-quality structures are only now becoming as well understood as their terrestrial counterparts. The improvements are due to both high-quality images and topography (now available for the Martian faults) and to the application of recent advances in understanding faults on Earth, such as displacement-length scaling relations. Here I review the major types of brittle structures seen on Mars and discuss their main characteristics and interpretations.

Normal faults and grabens are widely exposed on Mars, demonstrating crustal extension of large spatial extent but predominantly small magnitudes (less than a few percent strain). Areas of focused, localized extension are well known from many areas of Mars including the Valles Marineris and Tempe rift systems in the hemisphere-scale Tharsis volcanotectonic province. The segmented and echelon normal faults reveal clear displacement anomalies associated with their mechanical interaction and incipient segment linkage. Thrust faults also occur as spatially extensive sets, both as surface breaking faults and as blind thrust faults topped by forced "fault propagation" folds ("wrinkle ridges") at the planetary surface. Mechanical models have supplanted the earlier kinematic models as reliable bases for interpreting these structures. Strike-slip faults are present yet rather subtle on Mars. Rather than forming plate boundaries (that do not exist on one-plate Mars), the strike-slip faults define localized arrays of distributed horizontal shear strain. Deformation bands may also occur in the soft porous Martian sediments, redirecting subsurface fluid flow and contributing to ponding of water in transient lakes. All these structures attest to a complex and fascinating history of deformation throughout Mars? recorded 4 Ga history.